Whole Wheat Bread need help with Recipe

The elevation here is over just 5000 feet, I'm having a problem with whole wheat bread rising,[Recipe on Gold medal 5 # sack all purpose unbleached whole wheat flour ,like 4 cups whole wheat unbleached and 3 cups white unbleached flour, makes 2 loaves calls for 2 packets yeast.I got a good first rise, after knocking it down and putting it in the 2 loaf pans , plus another rise , doubled, but it collapsed in the oven,to about 4 inches high, and flat across the top. looked like hog feed ,but was not doey in the middle, and I didnt throw it out. Next batch,I used 1/3 less yeast, good rise , did double ,then no spring rise .Both loaves collapsed in the middle like a swayback horse , the ends of each loaf stayed up, But it was fluffy nice texture, good flavor, nice and brown and tasted good,sure as hell wouldn't win any prize to look at it.How do I get a nice fluffy loaf of whole wheat bread that stands up tall like the whole wheat bread at the great harvest bread co.Why I am doing this is I like whole wheat bread smells and tastes better than white bread.Thanks to all you bakers ; Leo